Add AsRef/AsMut<Headers> for Request/Response

[yoshuawuyts]

Adds {AsRef,AsMut}<Headers> impls for Request and Response. This is useful for generic functions that want to operate on http_types::Headers. With this Tide's req/res types can be passed directly to functions that operate on http-types's types:

fn set_custom(headers: impl AsMut<http_types::Headers>) {
    headers.insert("x-custom", "meow");
}

set_custom(&mut req);
set_custom(&mut res);

[jbr]

👍

[Fishrock123]

Should this kind of thing be done for others, e.g. Body as well?

[yoshuawuyts]

@Fishrock123 I was thinking probably not for Body -- we already forward AsyncRead -- but perhaps for Url, StatusCode and Method as well. But the right way to go about those is to add them to http-types first, and then forward them from tide second.